[og12] amdgcn: Use FLAT addressing for all functions with pointer arguments

The GCN backend uses a heuristic to determine whether to use FLAT or
GLOBAL addressing in a particular (offload) function: namely, if a
function takes a pointer-to-scalar parameter, it is assumed that the
pointer may refer to "flat scratch" space, and thus FLAT addressing must
be used instead of GLOBAL.

I came up with this heuristic initially whilst working on support for
moving OpenACC gang-private variables into local-data share (scratch)
memory. The assumption that only scalar variables would be transformed in
that way turned out to be wrong.  For example, prior to the next patch in
the series, Fortran compiler-generated temporary structures were treated
as gang private and moved to LDS space, typically overflowing the region
allocated for such variables.  That will no longer happen after that
patch is applied, but there may be other cases of structs moving to LDS
space now or in the future that this patch may be needed for.

2022-10-14  Julian Brown  <julian@codesourcery.com>

gcc/
	* config/gcn/gcn.cc (gcn_detect_incoming_pointer_arg): Any pointer
	argument forces FLAT addressing mode, not just
	pointer-to-non-aggregate.

diff --git a/gcc/ChangeLog.omp b/gcc/ChangeLog.omp
index d296eb137e8985afa0e32f146538f19edb51cf65..ceed4da97995967a578aba9fc4fd2ae1a90d9123 100644 (file)
--- a/gcc/ChangeLog.omp
+++ b/gcc/ChangeLog.omp
@@ -1,3 +1,9 @@
+2022-10-14  Julian Brown  <julian@codesourcery.com>
+
+       * config/gcn/gcn.cc (gcn_detect_incoming_pointer_arg): Any pointer
+       argument forces FLAT addressing mode, not just
+       pointer-to-non-aggregate.
+
 2022-10-12  Andrew Stubbs  <ams@codesourcery.com>
 
        * config/gcn/gcn.cc (gcn_expand_builtin_1): Change gcn_full_exec_reg

diff --git a/gcc/config/gcn/gcn.cc b/gcc/config/gcn/gcn.cc
index 1f8d8e199713f8bec33cdea485b944b49b6e7104..b01131c0dc2a345d72948db8786a5679dffcce3c 100644 (file)
--- a/gcc/config/gcn/gcn.cc
+++ b/gcc/config/gcn/gcn.cc
@@ -2819,10 +2819,14 @@ gcn_arg_partial_bytes (cumulative_args_t cum_v, const function_arg_info &arg)
   return (NUM_PARM_REGS - cum_num) * regsize;
 }
 
-/* A normal function which takes a pointer argument (to a scalar) may be
-   passed a pointer to LDS space (via a high-bits-set aperture), and that only
-   works with FLAT addressing, not GLOBAL.  Force FLAT addressing if the
-   function has an incoming pointer-to-scalar parameter.  */
+/* A normal function which takes a pointer argument may be passed a pointer to
+   LDS space (via a high-bits-set aperture), and that only works with FLAT
+   addressing, not GLOBAL.  Force FLAT addressing if the function has an
+   incoming pointer parameter.  NOTE: This is a heuristic that works in the
+   offloading case, but in general, a function might read global pointer
+   variables, etc. that may refer to LDS space or other special memory areas
+   not supported by GLOBAL instructions, and then this argument check would not
+   suffice.  */
 
 static void
 gcn_detect_incoming_pointer_arg (tree fndecl)
@@ -2832,8 +2836,7 @@ gcn_detect_incoming_pointer_arg (tree fndecl)
   for (tree arg = TYPE_ARG_TYPES (TREE_TYPE (fndecl));
        arg;
        arg = TREE_CHAIN (arg))
-    if (POINTER_TYPE_P (TREE_VALUE (arg))
-       && !AGGREGATE_TYPE_P (TREE_TYPE (TREE_VALUE (arg))))
+    if (POINTER_TYPE_P (TREE_VALUE (arg)))
       cfun->machine->use_flat_addressing = true;
 }